Analysis of protein-protein binding gives us a lot of useful information for studying functions of each protein. Analysis of binding between a compound available for a drug and protein gives us important information for revealing influences of such a compound on living organisms.
For analyzing the binding between a substance and protein, not only the specificity of the protein which binds to the substance, and but also the binding ability between the substance and protein (also referred to as the “binding activity”), are also important indicators.
For example, when an inhibitor against an enzyme is developed, if the binding ability of the inhibitor to the enzyme is lower than the binding ability of the original substrate of the enzyme to the enzyme, the inhibitor cannot fully function as an inhibitor.
The binding ability between molecules is generally represented by the binding constant (or dissociation constant). When two substances to be bound together are in an antigen-antibody relationship, the binding constant can be calculated by ELISA or RIA. Even when two substances to be bound together are in a relationship other than the antigen-antibody relationship, the binding constant can be obtained by equilibrium dialysis, fluorescence quenching, atomic force microscope, quartz vibrator, surface plasmon resonance, frontal affinity chromatography or other methods (see document (1) document (2)). However, in order to use these methods, it is required that both of the substances should be already known and that both of the substances should be purified and isolated before measurement. Therefore, these methods cannot be used for studying the binding ability of a substance to each of proteins contained in a mixture of plural kinds of unknown proteins.    (1) Borch, J., Roepstorff, P., Anal. Chem. 2004, 76, 5243-5248.    (2) Zhang, B., Palcic, M. M., Schriemer, D. C., Alvarez-Manilla, G, Pierce, M., Hindsgaul, O., Anal. Biochem. 2001, 299, 173-182.